The present invention relates to a system for transferring a toned image directly from a photoconductor to paper or transparency material using a liquid toner electrophotographic process, and in the absence of a liquid toner carrier on the transfer medium (e.g., paper).
Electrophotographic Processes are known which employ dry powder toners; other processes employ liquid toners. The principle difference between these two toners is how the pigment/binder particles are charged and the state of the carrier.
In dry powder toners, the charge is created by triboelectric charging (e.g., contacting polymers of different types to create charge exchange). The carrier in this case can be just a surface, roller, or particulate in nature. The developer mechanism which handles the toner provides the necessary contact for charging.
In liquid toners, charge control agents are chemically added to the carrier containing the pigment/binder components. The subsequent chemical reaction between the charge control agents and pigment/binder components results in the charging of the pigment/binder components. The liquid toner remains electrically neutral; however, once an electric field is applied both polarities migrate toward the opposite electrodes. This process is called electrophoresis. This is why previous transfer devices used carrier-wetted paper to allow for the mobility of the toner particles and counter charges. Images dried completely become neutral as there is no charge mobility without the liquid carrier. By using thermo-electric transfer of very dry images, some improvement is observed when utilizing electrical fields to aid thermal transfer over strictly a thermal approach. However, very dry images do not have enough charge mobility to provide the 100% transfer necessary for color image transfer. To obtain the appropriate color densities and hue, it is not only necessary to develop to appropriate densities for each hue, but it is also necessary to transfer consistent color densities to ensure that red remains red, green remains green, and so on. With less than 100% transfer, consistent color densities will not be transferred.
"Liquid toner" is a term that generally includes both the pigment/binder system and the insulating liquid; the insulating liquid is also referred to as the "liquid carrier." The liquid carrier may, for example, comprise paraffinic solvent blends of aliphatic, cycloalipatic, aromatic and halogenated hydrocarbons. These liquids are chosen because they are non-polar and, consequently, have a low dielectric constant and high electrical resistivity. Commercially available liquid toners include those sold under the trademarks "ISOPAR" and "NORPAR" by Exxon Corporation.
Direct transfer to paper in the absence of the liquid toner carrier has not been attempted by others. There has been electrostatic transfer from photoconductor to paper in the prior art; however, the paper was "wetted" with a liquid toner carrier. Insofar as in known, thermo-electric transfer of very dry images has not heretofore been attempted. "Very dry images" contain no less than 50% solids. Typically, in liquid development systems the "image areas," e.g., the areas in which pigment/binder are electrostatically developed, contain approximately 25% solids and 75% liquid carrier. However, with subsequent drying techniques, images with no less than 50% solids can be attained.
The advantage of direct, thermo-electric transfer is process simplicity and, thereby, product cost. Previous implementations either ignored the unacceptable problem of transferring wet images to paper, or used an intermediate transfer technique, a more complex and expensive approach. An intermediate transfer system is a transfer system whereby he image developed onto the photoconductor is transferred to an intermediate belt or roller mechanism and, subsequently, to paper. This results in two separate transfers which can utilize electrostatics or thermal processes.